Solid state klystron

ABSTRACT

A solid state Klystron structure is fabricated by forming a source contact and a drain contact to both ends of a conducting wire and by forming a bias gate and a signal gate on the conducting wire. The conducting wire may be at least one carbon nanotube or at least one semiconductor wire with long ballistic mean free paths. By applying a signal at a frequency that corresponds to an integer multiple of the transit time of the ballistic carriers between adjacent fingers of the signal gate, the carriers are bunched within the conducting wire, thus amplifying the current through the solid state Klystron at a frequency of the signal to the signal gate, thus achieving a power gain.

RELATED APPLICATION

This application is a divisional of U.S. Ser. No. 11/870,875, filed Oct.11, 2007, the entire contents of which are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates to semiconductor structures and method ofmanufacturing and operating the same, and particularly to solid stateKlystron structures and methods of manufacturing and operating the same.

BACKGROUND OF THE INVENTION

With the realization that end of scaling for conventional complementarymetal-oxide-semiconductor (CMOS) integrated circuits is fast approachingin the semiconductor industry, alternative nanostructures and materialshave been investigated. Of such nanostructures and materials, carbonnanotubes (CNTs) offer excellent intrinsic properties that are suitablefor high performance nanoscale devices.

A key advantage of CNTs over conventional CMOS devices is that scalinglimitations of metal-oxide-semiconductor field effect transistors(MOSFETs) due to boundary scattering of electrons from imperfectinterfaces are solved naturally in CNTs which have a smooth, wellcoordinated graphene structure with no bonds to the outside. Thisenables CNTs to retain excellent transport properties to much smallerlateral dimensions than silicon. The small radius and possibility ofcompletely surrounding the CNT by a gate provide excellent electrostaticconfinement of channel electrons, enabling the channel length to bescaled down to very small dimensions, and their small size would enablehigh packing densities. Band structure calculations of CNTs according toP. Avouris and J. Chen, “Nanotube electronics and optoelectronics,”Materials Today, 9, 46 (2006) show that conduction and valence bands aremirror images of each other, i.e., both electrons and holes should shareequally good transport properties. This indicates suitability of CNTsfor a general-purpose high-performance complementary circuit technology.

As is now well known, CNTs can be either metallic or semimetallic,depending on their chirality and have a bandgap which is inverselyproportional to their diameter for the semiconducting tubes. Theidealized electron/hole dispersion relation is hyperbolic in shape, witha quasi parabolic “effective mass” regime at lower energies and a linear“constant velocity” regime at higher energies, where the limitingvelocity, ν_(lim), is ˜5-10×10⁷ cm/sec according to G. Pennington and N.Goldsman, “Semiclassical transport and phonon scattering of electrons insemiconducting carbon nanotubes,” Phys. Rev. B 68, 045426 (2003).

Transport properties are further enhanced by the weak coupling of thecharge carriers to acoustic phonons and the fact that the opticalphonons have large energies of ˜0.15 eV. All of these factors lead toextraordinarily large mobilities, reported at ˜10⁵ cm²/V-sec at roomtemperature by Perebeinos et al., “Electron-Phonon Interaction andTransport in Semiconducting Carbon Nanotubes,” Phys Rev. Lett. 94,086802 (2005).

The intrinsic properties of CNTs make them good candidates for ballistictransport, and several signatures for ballistic transport have beenfound, for example, in Javey et al., “High-Field QuasiballisticTransport in Short Carbon Nanotubes,” Phys. Rev. Lett., 92, 106804(2004). The excellent ballistic transport properties of CNTs may besurpassed only by the ballistic transport properties of electrons invacuum tubes.

According to U.S. Pat. No. 2,242,275 to Varian, a vacuum tube Klystronas an ultra-high frequency amplifier is disclosed. The '275 patent isincorporated herein by reference to illustrate the operating principleof a vacuum tube Klystron.

Referring to FIG. 1, a schematic diagram demonstrating the operatingprinciples of a vacuum tube Klystron is shown. The vacuum tube Klystronconsists of an electron stream within a vacuum enclosure (not shown),confined by a magnetic filed to a tight beam. Electrons are emitted fromthe cathode by thermionic emission and are accelerated by the positivepotential V₀ at the anode. A series (two or more) of microwave cavitiesare placed along the electron beam so that the microwave electric fieldmodulates the velocity of the electron stream and causes bunching of theelectrons. This results in a strongly amplified current and is the basisfor the success of the vacuum tube Klystron as an ultra-high frequencyamplifier and generator with extensive use in radar applications.

Obviously, vacuum tube Klystrons are bulky vacuum tube devices thatcannot be easily integrated with solid state devices in a circuitdespite their excellent ultra-high frequency amplificationcharacteristics.

Therefore, there exists a need for a solid state ultra-high frequencyamplifier device that utilizes ballistic transport property of asemiconductor material and methods of operating the same.

SUMMARY OF THE INVENTION

The present invention provides a solid state Klystron (SSK) thatutilizes ballistic transport properties of a semiconductor material toamplify current at an ultra-high frequency of approximately 1 THz. Thecorrelated nature of ballistic transport is utilized to achieveamplification at frequencies well beyond transit-time limitations.

Specifically, the present invention provides a carbon nanotube solidstate Klystron (CNT SSK) that utilizes ballistic transport properties ofcarbon nanotubes to amplify current at an ultra-high frequency.Furthermore, the present invention provides a semiconductor solid stateKlystron (SSSK) that utilizes semiconductor material with long ballisticmean free paths instead of carbon nanotubes for ballistic transport ofelectrons or holes.

According to a first embodiment of the present invention, a carbonnanotube solid state Klystron (SSK) comprises:

at least one carbon nanotube;

a source contact to the at least one carbon nanotube;

a drain contact to the at least one carbon nanotube;

a signal gate with at least two signal fingers, wherein the at least twosignal fingers are located on the at least one carbon nanotube andbetween the source contact and the drain contact; and

a bias gate with at least one bias finger, wherein the at least one biasfinger is located on the at least one carbon nanotube.

Preferably, the at least one bias finger is located between the at leasttwo signal fingers. In one case, each of the at least one bias fingermay be located between two signal fingers and the number of the at leasttwo signal fingers is greater than the number of the at least one biasfingers by at least one, and preferably by one. In another case, theremay be an additional bias finger that is located outside the at leasttwo signal fingers, and the number of the at least two signal fingers isequal to or less than the number of the at least one bias fingers.

Preferably, the at least one carbon nanotube comprises a semiconductingnanotube. More preferably, each of the at least one carbon nanotube is asemiconducting nanotube.

Preferably, each of the source contact, the drain contact, the bias gateand the signal gate comprise a metal.

For optimal amplification of current, a width of each of the at leastone signal finger is equal to or less than about ½ of the smallestspacing between adjacent signal fingers, and is preferably equal to orless than about ⅓ of the smallest spacing between adjacent signalfingers.

The diameter of the at least one carbon nanotube is in the range fromabout 1.0 nm to about 10 nm, and preferably in the range from about 1.7nm to about 3 nm.

According to a second embodiment of the present invention, asemiconductor solid state Klystron (SSSK) comprises:

at least one semiconductor wire;

a source contact to the at least one semiconductor wire;

a drain contact to the at least one semiconductor wire;

a signal gate with at least two signal fingers, wherein the at least twosignal fingers are located on the at least one semiconductor wire andbetween the source contact and the drain contact; and

a bias gate with at least one bias finger, wherein the at least one biasfinger is located on the at least one semiconductor wire.

The at least one semiconductor wire comprises a material with longballistic mean free paths, which is preferably on the order of thelength of the at least one semiconductor wire. For example, the at leastone semiconductor wire may comprise a material selected from the groupconsisting of InAs and In_(x)Ga_(1−x)As, wherein x is in the range fromabove 0 to below 1.

Preferably, the at least one bias finger is located between the at leasttwo signal fingers. In one case, each of the at least one bias fingermay be located between two signal fingers and the number of the at leasttwo signal fingers is greater than the number of the at least one biasfingers by at least one, and preferably by one. In another case, theremay be an additional bias finger that is located outside the at leasttwo signal fingers, and the number of the at least two signal fingers isequal to or less than the number of the at least one bias fingers.

Preferably, each of the source contact, the drain contact, the bias gateand the signal gate comprise a metal.

For optimal amplification of current, a width of each of the at leastone signal finger is equal to or less than about ½ of the smallestspacing between adjacent signal fingers, and is preferably equal to orless than about ⅓ of the smallest spacing between adjacent signalfingers.

The width of each semiconductor wire has to be narrow enough so that thetransport along the wire is essentially one-dimensional in nature i.e.the electron motion is collimated along the length of the wire.Preferably, the width of the at least one semiconductor wire is in therange from about 2.0 nm to about 40 nm and the thickness of the at leastone semiconductor wire is in the range from about 2.0 nm to about 40 nm.

According to the present invention, a method of fabricating a carbonnanotube solid state Klystron (CNT SST) comprises:

forming a source contact and a drain contact;

forming at least one carbon nanotube, wherein the source contact and thedrain contact are in contact with the at least one carbon nanotube

forming a thin dielectric layer, wherein the thin dielectric layer is incontact with the at least one carbon nanotube;

forming a signal gate with at least two signal fingers, wherein the atleast two signal fingers are in contact with the thin dielectric layer;and

forming a bias gate with at least one bias finger, wherein the at leastone bias finger is in contact with the thin dielectric layer.

The source contact and drain contact may be formed prior to theformation of the at least one carbon nanotube. Alternatively, the sourcecontact and drain contact may be formed after the formation of the atleast one carbon nanotube.

The thin dielectric layer may be formed after the formation of the atleast one carbon nanotube and is deposited on and surrounds the at leastone carbon nanotube. At least one of the signal gate and the bias gatemay be formed thereafter.

Alternatively, the at least one of the signal gate and the bias gate maybe formed prior to the formation of the thin dielectric layer followedby the formation of the at least one carbon nanotube.

The at least one carbon nanotube may be a single carbon nanotube or maybe multiple parallel carbon nanotubes.

According to the present invention, a method of operating a solid stateKlystron comprises:

providing a solid state Klystron having at least one conducting wire, asource contact to the at least one conducting wire, a drain contact tothe at least one conducting wire, a bias gate with at least one biasfinger, and a signal gate with at least two signal fingers;

applying a first DC voltage bias between the source contact and thedrain contact;

applying a second DC voltage bias between the source contact and thebias gate; and

applying an AC voltage signal to the signal gate.

The conducting wire may be a carbon nanotube or a semiconductor wire.The first DC voltage, the second DC voltage, and the spacing betweeneach adjacent signal fingers are adjusted such that the transit time ofcarriers between an adjacent pair of the at least two signal fingers isan integer multiple of the inverse of the frequency of the AC voltagesignal, which is the period of the AC voltage signal. If the signal gatehas at least three signal fingers, carrier transit times betweenadjacent signal fingers, which are the average amounts of time that thecarriers take to move from one signal finger to the next signal finger,are preferably substantially the same for all pairs of adjacent signalfingers.

The AC voltage signal is preferably an ultra-high frequency signal witha frequency higher than 1 GHz, more preferably higher than 100 GHz, andmost preferably higher than 1 THz. Carrier bunching is induced withinthe at least one conducting wire to produce power gain.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a prior art vacuum tube Klystron, whichis reproduced from www.answers.com/topic/klystron.

FIG. 2 is a structure of a 2-period (3 signal fingers) solid stateKlystron with one conducting wire according to the present invention.

FIG. 3 is a schematic cross-section of an exemplary carbon nanotubesolid state Klystron according to the first embodiment of the presentinvention along the plane of X-X′ in FIG. 2.

FIG. 4 is a schematic cross-section of an exemplary semiconductor wiresolid state Klystron according to the second embodiment of the presentinvention along the plane of X-X′ in FIG. 2.

FIG. 5 is a structure of a 2-period (3 signal fingers) solid stateKlystron with multiple conducting wires according to the presentinvention.

FIG. 6 is a spatial distribution of electron energy for forward movingelectrons at a frequency equal to the inverse of the transit timebetween adjacent signal fingers within the 2-period (3 signal fingers)solid state Klystron according to the present invention.

FIG. 7 is a spatial distribution of electron energy for backward movingelectrons at a frequency equal to the inverse of the transit timebetween adjacent signal fingers within the 2-period (3 signal fingers)solid state Klystron according to the present invention.

FIG. 8 is a simulated output current of the 2-period (3 signal fingers)solid state Klystron as a function of frequency.

DETAILED DESCRIPTION OF THE INVENTION

As stated above, the present invention relates to solid state Klystronstructures and methods of manufacturing and operating the same, which isnow described in detail with accompanying figures.

Referring to FIG. 2, a top-down view of an exemplary solid stateKlystron comprising a conducting wire 50, source contact 10, a draincontact 90, a bias gate 60 having two bias fingers 61, and a signal gate40 having three signal fingers 41 is shown. The carriers, e.g.,electrons or holes, flow from the source contact 10, through theconducting wire 50, to the drain contact 90 in a ballistic transport byan applied suitable first DC voltage bias between the drain contact 90and the source contact 10. To facilitate the ballistic transport of thecarriers, the material for the conducting wire 50 is chosen such thatballistic mean free paths within the conducting wire 50 are long, thatis, comparable to the dimension of the length of the conducting wire 50.The carriers are provided by a suitable second DC voltage bias betweenthe bias gate 60 and the source contact 10.

The first DC voltage bias across the drain contact 90 and the sourcecontact 10 is on the order of or less than 1.0 V to prevent loss ofballisticity through emission of ballistic phonons. Likewise, the secondDC voltage bias across the bias gate 60 and the source contact 10 isalso on the order of 1.0 V. However, the second DC voltage bias mayexceed 1.0V as needed. The second DC voltage bias may exceed, may be thesame as, or may be less than the second DC bias voltage.

According to the present invention, the minimum number of signal fingers41 of the signal gate 40 is two. More than two signal fingers 41 may beemployed to increase the gain of the SSK. The signal fingers 41 arelocated on the conducting wire 50. The signal fingers 41 contact a thindielectric layer (not shown in FIG. 2) which contacts the conductingwire 50. The signal fingers 41 may be located on one side of theconducting wire 50 or they may surround the conducting wire 50. Thecarriers, that is, electrons or holes, are confined within theconducting wire 50 and do not flow into the bias gate 60 or into thesignal gate 40 due to the thin dielectric layer.

According to the present invention, the spacing between adjacent signalfingers 41 of the signal gate 40 are adjusted such that the carriervelocity is in resonance with the AC voltage signal applied to thesignal fingers 41, that is, carriers that are accelerated by the ACvoltage signal at the first signal finger (the signal finger closest tothe source contact 10) are again accelerated by the AC voltage signal atthe next signal finger. This is achieved by matching the transit time ofcarriers between an adjacent pair of the signal fingers 41 to an integermultiple of the period of the AC signal, that is, the inverse of thefrequency of the AC voltage signal. If the signal gate 40 has at leastthree signal fingers 41, preferably, all transit times between adjacentpairs of the signal fingers 41 are integer multiples of the inverse ofthe frequency of the AC voltage signal. Most preferably, carrier transittimes between adjacent signal fingers 41 are substantially the same forall pairs of adjacent signal fingers.

For optimal amplification of current, the width Ws of the signal fingers41 should not exceed the width of the ‘bunched” carriers, or the pulsewidth of the carriers. Therefore, the width Ws of the signal fingers 41is equal to or less than about ½ of the smallest spacing betweenadjacent signal fingers 41, and preferably equal to or less than about ⅓of the smallest spacing between adjacent signal fingers 41.

According to the present invention, at least one bias finger 61 islocated between a pair of adjacent signal fingers 41. The number of thesignal fingers 41 may be greater than the number of the bias fingers 61by at least one. Preferably, one bias finger 61 is located between eachpair of adjacent signal fingers 41, and therefore, the number of thesignal fingers 41 is greater than the number of the bias fingers 41 byone.

According to the first embodiment of the present invention, theconducting wire 50 is a carbon nanotube, and is preferably asemiconducting carbon nanotube. In this embodiment, the room temperaturemean free path of carriers in a carbon nanotube is about 1 micron, or1,000 nm, and correspondingly, the distance between the source contact10 and the drain contact 90 in a carbon nanotube SSK for a roomtemperature operation may be up to about 1 micron. At low temperatures,acoustic phonons are suppressed, thereby increasing the mean free pathof carriers. Consequently, the distance between the source contact 10and the drain contact 90 in a carbon nanotube SSK for a low temperatureoperation may exceed 1 micron and may be above 10 microns.

FIG. 3 shows a vertical cross-section of an exemplary carbon nanotubesolid state Klystron (CNT SSK) according to the first embodiment of thepresent invention along the direction of X-X′ in FIG. 2. According to anexemplary process for the formation of the CNT SSK, an insulating layer100 may be formed on a semiconductor substrate or on an insulatorsubstrate. Alternatively, the insulating layer 100 may be the substrate.Non-limiting exemplary materials for the insulating layer 100 includequartz (SiO₂), deposited silicon oxide, aluminum oxide, and siliconnitride. The source contact 10 and the drain contact 90 may be formed atthis stage by deposition of a metal and lithographic patterning. Acarbon nanotube 50A is formed on the source contact 10 and the draincontact 90 utilizing techniques well known in the art. A thin dielectriclayer 110 is formed on and surrounds the carbon nanotube 50A. The thindielectric layer 110 may be, for example, an aluminum oxide layerdeposited by an atomic layer deposition (ALD) at a thickness of about 10nm. Bias fingers 61 and signal fingers 41 are formed directly on thethin dielectric layer 110, preferably by deposition of a gate metallayer and lithographic patterning.

Alternatively, the source contact 10 and the drain contact 90 may beformed after the gate metal is formed. In this case, two end portions ofthe thin dielectric layer 110 are etched to expose the carbon nanotube50A and a layer of metal is deposited and patterned over the exposedportions of the carbon nanotube to form the source contact 10 and thedrain contact 90. In another implementation, at least one of the signalgate 40 and the bias gate 60 may be formed prior to forming the carbonnanotube 50A.

The fabrication of a single conducting wire follows well establishedlithographic procedures such as deep ultraviolet (DUV) lithography. Forthe finest finger to finger pitch, however, electron beam lithographymay be utilized instead of DUV lithography. In an exemplary case of aCNT SSK with a three finger signal gate 40 and a two finger bias gate60, which has an equal spacing between each adjacent pair of the signalfingers 41 and between each adjacent pair of the bias fingers 61, and anequal width for each of the signal fingers 41 and for each of the biasfingers 61 such that the width of the signal fingers 41 and the biasfingers 61 is ⅓ of the spacing between a pair of adjacent signal fingers41, the distance between the source contact 10 and the drain contact isequal to the sum of the distance from the source to the first sourcefinger, 9 times the width Ws of one of the signal fingers 41, and thedistance from the last source finger to the drain contact. If the widthWs of one of the signal fingers 41 is about 40 nm, the length of the CNTSSK can be about 500 nm, which is well less than the length of ballisticmean free path of carriers in a carbon nanotube at room temperature.

According to the second embodiment of the present invention, theconducting wire 50 is a semiconductor wire which comprises asemiconductor material with a long carrier mean free path. For example,the semiconductor wire may comprise InAs and In_(x)Ga_(1−x)As with thevalue of x ranging from above 0 to below 1, which has a relatively longmean free path of about 50 nm at room temperature. The distance betweenthe source contact and the drain contact is preferably less than theroom temperature mean free path in a semiconductor SSK for roomtemperature operation. As in the first embodiment, the distance betweenthe source contact and the drain contact may be increased for asemiconductor SSK for a low temperature operation.

FIG. 4 shows a vertical cross-section of an exemplary semiconductorsolid state Klystron (SSSK) according to the second embodiment of thepresent invention along the direction of X-X′ in FIG. 2. According to anexemplary process for the formation of the SSSK, an insulating layer 100is formed on a semiconductor substrate or on an insulator substrate.Alternatively, the insulating layer 100 may be the insulating substrate.A semiconductor material with a long ballistic mean free path, forexample, In_(x)Ga_(1−x)As with the value of x ranging from above 0 tobelow 1, and preferably about 0.4, is deposited and lithographicallypatterned to form a semiconductor wire 50B. The width of thesemiconductor wire 50B is preferably in the range from about 2.0 nm toabout 40 nm and the thickness t of the semiconductor wire 50B is in therange from about 2.0 nm to about 40 nm. A thin dielectric layer 110 isformed over the semiconductor wire 50B. The thin dielectric layer 110may, for example, comprise an In_(y)Al_(1−y)As layer, with the value ofy ranging from above 0 to below 1, and preferably about 0.6, accordingto the second embodiment. A first conducting material, preferably ametal with high conductivity, is deposited and patterned to form thebias fingers 61 and the signal fingers 41 directly on the insulatinglayer 100. Portions of the thin dielectric layer 110 are etched toexposed the underlying semiconductor wire 50B near the ends, followed bya deposition of a second conducting material and lithographic patterningto form a source contact 10 and a drain contact 90.

While the conductor wire 50 is formed prior to the formation of the biasgate and the signal gate in the exemplary processes, one skill in theart would recognize that the order may be reversed in both embodimentswith minor structural modifications. Such alterations in the order ofprocessing and resulting changes in the structures of CNT SSK and SSSKin both embodiments of the present invention are herein explicitlycontemplated.

Furthermore, each of the signal gate 40 and the bias gate 60 may beformed prior to the formation of the conductive wire 50 or after theformation of the conductive wire 50 as long as a thin dielectric layer110 is formed between the conductive wire 50 and each of the signal gate40 and the bias gate 60. If desired, the signal gate 40 and the biasgate 60 may be formed separately. Multiple thin dielectric layers 110may be utilized as necessary.

While the gain of the SSK may be increased by adding more signal fingersto the SSK, the distance between the source contact 10 and the draincontact 90 is limited due to the finite ballistic mean free paths.Furthermore, the parasitic capacitance of the SSK also increases withincreasing number of signal fingers 41. To increase the output power andto reduce the effect of parasitic resistance, multiple conducting wiresare used as shown in FIG. 5.

Such a multiple conducting wire configuration is possible in both thefirst and the second embodiments. According to the first embodiment,typical power levels of about 1 μW per carbon nanotube are possiblebased on peak signal amplitude of about 0.1V and DC current levels ofabout 10 μA. About 100 parallel carbon nanotubes would be able to matcha 50 Ohm load and give power output levels of about 1 mW. The carbonnanotubes need to be well matched in their characteristics in order thatthe currents in different carbon nanotubes be in phase with each other.

In terms of the operating principle, the carbon nanotube solid stateKlystron (CNT SSK) according to the first embodiment of the presentinvention and the semiconductor solid state Klystron (SSSK) according tothe second embodiment of the present invention are direct descendents ofvacuum tube Klystrons, where ballistic transport is the norm, but on avastly smaller scale. A conducting wire 50 that allows a ballistictransport of carriers within, such as a carbon nanotube 50A or asemiconductor wire 50B, replaces the vacuum tube. The walls of thecarbon nanotube or the walls of the semiconductor wire confine theelectrons radially, so that a confining magnetic field is not necessary.The source contact 10 and the drain contact 90 replace the cathode andanode of a vacuum tube Klystron respectively. The multi-finger signalgate 40 replaces the microwave cavities. The bias gate 60 betweenfingers create drift region. The bias gate 60 is functionally analogousto the anode of a vacuum tube Klystron. Because of the close proximityof the multiple fingers of the signal gate 40, the longitudinal fielddeveloped between the signal fingers 41 and the bias fingers 61 issufficiently large that no resonant cavity is required here. Even thoughelectrons move much slower in carbon nanotubes 50A or in semiconductorwires 50B compared with a vacuum tube (by a factor of about 100×), thedimension between the source contact 10 and the drain contact 90 of thecarbon nanotube 50A or of the semiconductor wire 50B is at leastthousands of times smaller than the dimension between the cathode andthe collector of a vacuum tube Klystron, enabling ultra-high frequencyamplification of signals.

Simulation of the SSK according to the present invention shows that thekinetic energy in the channel may be increased beyond the applied gatevoltages by suitably timing the gate signals with respect to theballistic transit-time between gates. When distances between signalfingers 41 of the signal gate 40 are less than 1 μm, terahertzamplification may be possible.

Referring to FIG. 6, a spatial distribution of electron energy withinthe 2-period (3 signal fingers) solid state Klystron according to thepresent invention is shown for forward moving electrons. The frequencyof the AC voltage signal applied to the signal gate 40 has a frequencyequal to the inverse of the transit time between a pair of adjacentsignal fingers 41, i.e., the period of oscillation is equal to thetransit time between a pair of adjacent signal fingers 41. The potentialenergy imparted to the electrons due to the AC voltage signal is shownby the potential energy curve 200 in FIG. 6. As electrons travel fromthe source contact 10 to the drain contact 90, the kinetic energy of theelectrons increases and bunching of electrons occurs. The same result isobtained for a mirror image implementation in which holes are used ascarriers. The added kinetic energy may be much larger than the potentialenergy imparted by the signal gate 40, in analogy to high energyparticles in a particle accelerator. The carrier bunching is the sourceof the power gain mechanism in the Klystron.

Some carriers may travel in the opposite direction of the current flowduring operation of an SSK. Referring to FIG. 7, a spatial distributionof electron energy within the 2-period (3 signal fingers) solid stateKlystron according to the present invention is shown for backward movingelectrons. The potential energy imparted to the electrons due to the ACvoltage signal is shown by the potential energy curve 200 in FIG. 7. Thefraction of backward moving electrons among all electrons in theelectron beam is a function of the design parameters of the SSK,including the amplitude of the AC voltage signal relative to the firstDC voltage bias and relative to the second DC voltage bias. Thisfraction may be contained to be less than 0.1, and under optimalconditions may be contained to be less than 0.01. Further, the kineticenergy of the backward moving electrons is much less than the kineticenergy of the forward moving electrons. While the faction of thebackward moving electrons affect the effectiveness of an SSK, theoverall operation of the SSK is not altered by the backward movingelectrons.

Referring to FIG. 8, a simulated AC output current of the 2-period (3signal fingers) SSK according to the present invention is shown as afunction of frequency. The current is in relative units and thefrequency is in units of the inverse of the transit time between a pairof adjacent signal fingers 41. P_(scat) is a probability for backscattering normalized to the length of SSK, i.e., the distance betweenthe source contact 10 and the drain contact 90. If the distance betweena pair of adjacent fingers is one micron and the drift velocity of thecarriers is 10⁸ cm/sec, then the frequency scale would be in the THz.Thus, the SSK according to the present invention is capable of operationabove 1 THz.

The SSK according to the present invention offers a compact source ofradiation at a frequency above 100 GHz, and preferably above 1 THz. TheSSK according to the present invention can also be used as an ultra-highfrequency amplifier at a frequency higher than 1 GHz, more preferablyhigher than 100 GHz, and most preferably higher than 1 THz.

The number of signal fingers 41 and the number of bias fingers 61, thespacings and widths of the signal fingers 41 and the bias fingers, aswell as the geometry of the signal fingers 41, bias fingers 61, sourcecontact 10, and the drain contact 90 may be adjusted for optimaloperation of the SSK according to the present invention.

While the invention has been described in terms of specific embodiments,it is evident in view of the foregoing description that numerousalternatives, modifications and variations will be apparent to thoseskilled in the art. Accordingly, the invention is intended to encompassall such alternatives, modifications and variations which fall withinthe scope and spirit of the invention and the following claims.

What is claimed is:
 1. A method of fabricating a carbon nanotube solidstate Klystron (CNT SST), comprising: forming a source contact and adrain contact; forming at least one carbon nanotube after forming saidsource contact and said source contact, wherein said source contact andsaid drain contact are in contact with said at least one carbonnanotube; forming a thin dielectric layer, wherein said thin dielectriclayer is in contact with said at least one carbon nanotube; forming asignal gate with at least two signal fingers, wherein said at least twosignal fingers are in contact with said thin dielectric layer; andforming a bias gate with at least one bias finger, wherein said at leastone bias finger is in contact with said thin dielectric layer.
 2. Themethod of claim 1, wherein said thin dielectric layer is deposited onand surrounds said at least one carbon nanotube.
 3. The method of claim1, wherein said at least one carbon nanotube is multiple parallel carbonnanotubes.
 4. The method of claim 1, wherein said at least one carbonnanotube is a semiconductor nanotube.
 5. The method of claim 1, whereinsaid forming said source contact and said drain contact comprisesdepositing a metal and lithographic patterning said metal.
 6. The methodof claim 1, wherein said forming the thin dielectric layer comprisesatomic layer deposition to a thickness of about 10 nm.
 7. The method ofclaim 1, wherein said at least two signal fingers are formed bydeposition of a gate metal layer and lithographic patterning.
 8. Themethod of claim 1, wherein said at least one bias finger is formed bydeposition of a gate metal layer and lithographic patterning.
 9. Themethod of claim 1, wherein said source contact and said drain contactare formed on uppermost surface of an insulating layer.
 10. A method offabricating a carbon nanotube solid state Klystron (CNT SST),comprising: forming at least one carbon nanotube; forming a thindielectric layer, wherein said thin dielectric layer is in contact withsaid at least one carbon nanotube; forming a source contact and a draincontact, wherein said source contact and said drain contact are incontact with said at least one carbon nanotube; forming a signal gatewith at least two signal fingers, wherein said at least two signalfingers are in contact with said thin dielectric layer; and forming abias gate with at least one bias finger, wherein said at least one biasfinger is in contact with said thin dielectric layer.
 11. The method ofclaim 10, wherein said thin dielectric layer is deposited on andsurrounds said at least one carbon nanotube.
 12. The method of claim 10,wherein said at least one carbon nanotube is multiple parallel carbonnanotubes.
 13. The method of claim 10, wherein said at least one carbonnanotube is a semiconductor nanotube.
 14. The method of claim 10,wherein said forming said source contact and said drain contactcomprises etching two end portions of the thin dielectric layer toexpose two end portions of the at least one carbon nanotube, depositinga metal and lithographic patterning said metal.
 15. The method of claim10, wherein said forming the thin dielectric layer comprises atomiclayer deposition to a thickness of about 10 nm.
 16. The method of claim10, wherein said at least two signal fingers are formed by deposition ofa gate metal layer and lithographic patterning.
 17. The method of claim10, wherein said at least one bias finger is formed by deposition of agate metal layer and lithographic patterning.